It is known in the automotive industry to join two metal preformed panels into a unitary hollow structural unit. Typical units of this type include vehicle doors, hoods, and trunk lids. Collectively, these units are referred to as closure panels.
A conventional process of joining the two panels together is referred to as hemming and results in a flange of the outer panel being folded over and gripping the edge of the inner panel. The process is well known. A conventional method and apparatus for joining two metal preformed panels is disclosed in U.S. Pat. No. 5,150,508 and its disclosure is incorporated herein by reference.
However, a problem exists in applying the conventional hemming process to complex shapes used on some hoods and trunk lids. Usually the hemming process is used to hem a majority of the periphery of a pair of corresponding preformed panels. A portion of the periphery may be difficult to hem due to the requirement of complex die shapes or the requirement of a plurality of dies so that some of the periphery may be non-hemmable.
In such cases the pair of panels is removed from the hemming machine that imparts the hemmed edge and in a separate process, at a separate station, the non-hemmed portion is spot welded or extruding clinched to join the non-hemmed portion of the panels together.
Spot welding of the non-hemmed portion of the otherwise hemmed periphery of a closure panel works well with steel panels generally. However, the use of aluminum in many of today's closure panels creates additional problems as aluminum does not easily spot weld.